1. Field of the Invention
This invention relates to a carriage and an image forming device including a carriage arranged therein.
2. Description of the Related Art
In an ink jet type image forming device, a linear scale is arranged in a position corresponding to a movable range of a carriage which carries an ink discharging device. On the linear scale, a number of slits are formed at given intervals in the longitudinal direction of the linear scale. An encoder sensor is arranged on the carriage to read the slits on the linear scale. By using the encoder sensor to read the slits on the linear scale, the position information of the carriage in the main scanning direction is acquired.
In the image forming device, the transfer timing of image data and the discharge timing of the ink from the ink discharge head are determined based on the position information of the carriage, thereby carrying out the image formation with high quality.
However, if ink mist, paper dust, etc. adhere to the linear scale, it is difficult for the encoder sensor on the carriage to read correctly the slits on the linear scale, and an error may arise in the detected position information of the carriage.
As a result, the timing of discharging of the ink is shifted inappropriately, which will cause the deviation of a printed image to present. When the stain on the linear scale is severe, it is impossible to detect the position information of the carriage, which may cause the carriage to collide with the side plate of the image forming device, and the image forming device may be physically damaged.
For this reason, it is desired to acquire accurate position information of the carriage detected by the encoder sensor.
For example, Japanese Laid-Open Patent Publication No. 2005-349799 discloses an image forming device in which the mist of an ink discharged from the discharge head is charged, and the ink mist around the carriage is attracted and removed, so that the inside of the image forming device is kept clean and no stain is present.
In the image forming device disclosed in Japanese Laid-Open Patent Publication No. 2005-349799, a discharge head is arranged to include a charging electrode for charging the mist of the ink, and a dust-collecting electrode for attracting the mist of the charged ink. In this image forming device, the mist of the charged ink is collected by the dust-collecting electrode, and it is possible to prevent the mist of the ink from adhering to all the component parts of the image forming device including the linear scale.
Japanese Laid-Open Patent Publication No. 2006-272770 discloses an image forming device which is aimed at preventing the deviation of a printed image even when a stain is present partially on the linear scale. In this image forming device, when the carriage is moved at a fixed speed uniformly, the period of the output signals of the encoder sensor is checked. When an erroneous period of the output signals of the encoder sensor is detected, the linear scale is moved in the up/down direction of the linear scale (or the width direction of the linear scale). At this time, a clean portion of the linear scale in which no stain is present is found out, and the linear scale is moved to cause the encoder sensor to face the clean portion of the linear scale, so that the encoder sensor reads the slits on the linear scale. Thus, it is possible to acquire accurate position information of the carriage from the output signals of the encoder sensor.
The method of Japanese Laid-Open Patent Publication No. 2005-349799 uses the dust-collecting electrode for collecting the dust with the ink mist wherein the mist of the ink from the discharge head is charged. The method must be arranged to meet various conditions of the dispersing ink mist (the physical properties of the respective color inks, the mass of the ink mist, and the kinetic energy of the ink mist), and it is difficult to completely collect the dust with the ink mist. In some cases, the arrangement of the dust-collecting electrode may not be appropriate for prevention of the adhesion of the ink mist to the linear scale. For this reason, there is the problem that the remaining ink mist which cannot be collected by the dust-collecting electrode may adhere to the linear scale.
The method of Japanese Laid-Open Patent Publication No. 2006-272770 uses the movement of the linear scale which is large in the longitudinal direction to cause the encoder sensor to face the clean portion of the linear scale in which no stain is present. To perform the movement of the long linear scale in the up/down direction, the rigidity of the linear scale, the accuracy of the control of the driving source, etc. must be taken into consideration. If a mechanical deviation in the linear scale is present, the reliability of the reading of the slits on the linear scale by the encoder sensor will be insufficient.
In the image forming device of Japanese Laid-Open Patent Publication No. 2006-272770, the stain on the linear scale can be accurately detected by the encoder sensor only when the carriage is moved at the fixed speed. If the carriage is moved in an accelerating or decelerating state, the stain on the linear scale is not accurately detected by the encoder sensor. Due to inaccurate reading of the slits on the linear scale by the encoder sensor, a deviation of a printed image or overrunning of the carriage may arise.